What conceivable reason would Mibbit have for doing that?
Mibbit must be trusted to ... not allow proxy connections
No, that's not true. Mibbit must be trusted merely to pass-on the correct IP. Then Freenode's existing proxy detection can handle the job of rejecting proxied connections.
You are publicly calling into question Mibbit's integrity here, and suggesting that there's an incentive to give Freenode false information about users' IP addresses. Why?